The extrusion of molten polymers and copolymers, with subsequent bonding of the extruded polymer film or “curtain” onto a substrate, is well known. The process normally involves melting the polymer in an extruder, extruding the molten polymer through a die to form the melt curtain, and passing the substrate (e.g., paper or aluminum foil) and the extruded film simultaneously between two pressure rolls to bond the hot polymer film and the substrate. In a typical operation, plastic resin pellets are melted and extruded through a narrow (e.g., 0.030 inch) linear die opening to form the melt curtain or molten plastic web. Various polymers suitable for extrusion coating include polyolefins (e.g., polyethylene), olefinic copolymers (e.g., ethylene-propylene or ethylene-acrylic acid copolymers), and olefinic polymer blends (e.g., a blend of polypropylene with either polyethylene or polystyrene).
Extrusion coating is described in detail, for example, in U.S. Pat. No. 4,183,845 and WO 2002/094556. The process may be used in a number of applications, for example in the preparation of labels and decals or packaging materials such as those used in food packaging.
The adhesion of the molten polymer to the extrudate can be significantly improved with ozone application to the side of polymer melt curtain that contacts the substrate, or otherwise contacts an intermediate layer that adheres to the substrate. In particular, a discharge of ozone in the vicinity of the molten polymer, shortly before it contacts the substrate, provides controlled oxidation on the bonding or “preferred” side of the extrudate. The process known as melt curtain ozonation therefore provides bonding sites on the molten polymer which promote oxidative type bonding.
Good polymer/substrate adhesion and consequently minimal delamination is critically dependent on the manner in which ozone is applied. Also, the close proximity between the equipment from which ozone is applied (i.e., the ozone applicator) and the molten polymer renders this equipment subject to plugging and/or to becoming caked or coated, both internally and externally, with the polymer or polymer byproducts. This results in uneven ozone distribution and consequently poor adhesion of the finished laminate. Frequent cleaning (e.g., at approximately six-week intervals) of the ozone applicator is therefore often mandatory to combat the deposition and condensation of polymer, both onto and into, the ozone applicator, due to its contact with molten polymer or polymer vapors.
Thus, there remains a need in the art for improved gas distributors such as ozone applicators, as well as ozonation apparatuses and methods, which can improve the application of ozone in melt curtain ozonation, thereby reducing polymer/substrate delamination as well as the detrimental effects, including decreased operating efficiency, associated with polymer deposition on the ozone applicator.